The present invention is directed generally to a cable stay damper band and to its method of use. More specifically, the present invention is directed to a cable stay damper band that is usable with both new cables and as a retrofit to existing cables. Most particularly, the present invention is directed to cable stay damper bands that are securable to cable stays in an application pattern which significantly reduces fluid current induced, or other induced, vibrations in the cable. The cable stay damper bands are structured to be attached to or placed about both new cables as well as existing cables in a particular pattern or array. These cable stay damper bands can also be incorporated into the cable stay at the time of manufacture in order to eliminate the need for a later retrofit. Vibrations can be induced in a cable by the passage of any fluid, such as air or water, over the surface of a cable. The use of damper bands in accordance with the present invention has been very effective in the substantial reduction and near elimination of wind/rain induced vibrations in cable stays in air. Similarly, damper bands in accordance with the present invention can have a significant effect in the reduction of vibrations or oscillations induced in an underwater cable as a result of relative movement between the cable and the fluid; i.e. water in which the cable is placed. The damper bands, if intended to be placed on the exterior of the cable, have a circular, or ringed shape that will counteract these fluid passage induced vibrations in the cable stay generated by the relative movement of the cable and the fluid which surrounds it.
The damper bands can be either passive or active. Passive damper bands will function without any mechanical or electrical input. Active damper bands can be reactive or proactive and include movable masses. These movable masses can be caused to move in reaction to cable induced vibrations and are thus reactive. They can also be caused to shift by either mechanical or electrical devices and are thus proactive. The proactive damper bands with actuating devices can be considered to be computer-controlled devices. In the case of active damper bands, these can either form the generalized ring or ribbed shape on the cable stay exterior surface, or they can be embedded within the cable stay, thus, leaving a smooth outer cable stay surface. Typically, the distributed bands or rings will be placed along the full length of the cable stay. It is also possible for only a partial length of the cable, e.g. the top third or top quarter of cable stay length, to have the distributed damping devices attached.
The implementation of an active, smart underwater cable vibration damping system is included in the present invention. The active underwater system of the present invention employs distributed bands or rings together with small, embedded mechanical dampers, such as shiftable media, pendulums, and/or spring type inertial masses that may be energized using active smart control when the cable vibration reaches a threshold limit. Due to the extreme depths at which underwater cables, which can receive the damper bands of the present invention, are mounted, efforts in active smart control are focused on low-maintenance damping techniques and low-cost cable modifications.
The use of cable stays in the construction of a wide variety of structures is well known. Any number of types of bridges use various cables to support bridge decks, to hold bridge towers steady and to generally form the support for the bridges. Suspension bridges are one example of a bridge structure that uses a large number of elongated cables as stays and supports. In a somewhat similar manner, cables are frequently used as guy wires or as stays in connection with tall antenna towers and the like. A large number of these towers are used to support various receivers, repeaters and other similar assembles. One need not look far without seeing such a tower. A plurality of elongated cables are typically run from various elevations on these towers to suitable ground anchors. These cable stays or guy wires are used to stabilize the tower.
Elongated cables are also utilized in the underwater stabilization of off-shore structures such as floating oil drilling installations. These cables are subjected to hydrodynamic forces that are very similar to the aerodynamic forces which above ground stay cables and guy lines experience. A much slower fluid flow speed in water is capable of producing cable stay vibrations found generally only with very high wind speeds, i.e. low water speeds generally correspond to fluid flow in high air speeds.
In all of these cable applications, the passage of a fluid, such as air or water, over the surface of the cable, can induce vibrations or oscillations in the cable. If the fluid velocity is sufficient, the cable can be seen to vibrate at node points with sufficient amplitude that the structure with which the cable is associated may be damaged. In the case of cable stays for use with off-shore structures, the off-shore structural stays may be caused to vibrate or in extreme situations to shake sufficiently that the structural integrity of the off-shore structure may be compromised. Such vibrations can also cause fatigue in the cables. It is relatively common knowledge that fatigue in off-shore oil platforms, due primarily to underwater currents, is a problem. Such cable stay vibrations can be severe and have led to concerns that they are contributing to significant fatigue loads on the cables. At risk is the material that makes up the cable stay itself, as well as the anchorage devices. Such fatigue problems can lead to the need for early replacement of the cables. In the situation involving sub-sea cables, the position of the anchored platform can be affected with a resultant possible misalignment of platform supported drill strings and other similar down-hole implements.
For cable stays in air, it has been proposed to provide various mechanical vibration dampers for elongated cables. In one configuration, these vibration dampeners have taken the form of shock absorber-like devices that may be interposed between an end of the cable and an anchoring or attachment site for the cable. Other similar spring-biased connections have been used in the past in an effort to compensate for or to counteract wind induced vibrations and oscillations.
Fairings and streamlining devices have been applied to overhead cables, to sub-sea cables and to guy wires and cable stays. These attempt to altar the shape of the generally cylindrical cable to create an airfoil or flow-smoothing shape.
Various forms of underwater surface treatments have been sought to serve as solutions to the vibrations of the smooth-surfaced, circular cable. While some of these treatments can be adopted only at the cable design stage, others are feasible for retrofitting of the cables. Fluid dynamic countermeasures usually modify the surface of the cable cross section to improve its fluid dynamic performance in terms of reducing the excitation from the moving fluid, e.g. water, or increasing the fluid dynamic damping. Two examples of generally known types of underwater cable surface/cross section modifications are surface dimpling and parallel axial protuberances.
It is also known in the art to fabricate structures with integrally formed annular rings and with various projections and protrusions. In these structures, the rings are formed during the fabrication of the structure, which may be a mast of an outdoor antenna, a smokestack, transmission lines or pipelines. These rings are intended to reduce or to eliminate the vortex shedding which affects structures of these types. The elimination of this vortex shedding will greatly reduce the oscillating lateral forces which smokestacks, antennas, transmission lines and other cylindrical structure have been plagued by due to this periodic shedding of vortices.
The prior art has appreciated the use of various vibration dampers and integrally formed annular rings and bands as well as various fairings and spoilers. However, there continues to exist a need for cable stay dampers and for their method of use and application which will reduce or mitigate fluid induced cable vibrations and which will overcome the limitations of the prior art devices.
It is an object of the present invention to provide a cable stay damper band.
Another object of the present invention is to provide a method of using passive and active cable stay damper bands.
A further object of the present invention is to provide passive and active cable stay damper bands for retrofit use.
Still another object of the present invention is to provide a passive or active cable stay damper band which is effective in counteracting fluid current induced vibrations in cables positioned in air and in water.
Yet a further object of the present invention is to provide a passive or active cable stay damper band which damps both low and high modal vibrations.
Even still another object of the present invention is to provide a passive or active cable stay damper band which is economical to use and which is easily attached.
A further object of the present invention to provide a passive or active smart control system for cable stay fluid flow vibration mitigation using distributed circular bands or rings, where the outside surface of the cable stay has a resulting ribbed shape.
Still a further object of the present invention is to provide a smart control system of fluid flow induced vibration mitigation using damper bands with embedded mechanical dampers, where the outside surface of the cable stay retains a smooth shape.
Yet another object of the present invention is to provide fluid flow vibration mitigating damper bands or rings having embedded mechanical dampers that are energized using active smart control when cable vibrations reach a threshold value.
As will be set forth in greater detail in the description of the preferred embodiments which are presented subsequently, the fluid flow cable stay damper bands in accordance with the present invention, and their method of use are primarily intended to mitigate fluid flow induced vibrations in cable stays of structures. The present invention is directed to the mitigation of air current and underwater current induced vibrations of cable stays in structures such as bridges, towers, masts, off-shore oil platforms and the like.
The distributed fluid-dynamic and mechanical damping of vibrations induced in air or underwater situated cables through the utilization of active smart control, in accordance with the present invention, utilizes a plurality of damper bands or rings that are positionable at spaced lengths along the cable stay to be dampened. Each damper band, other than an active, embedded band, has an outer, fluid-dynamic shape and a hollow or partially hollow interior. The interior of each damper band is provided with active mechanical dampers. These can take the form of shiftable weights, pendulums, spring type inertial masses and other movable or shiftable bodies. In one embodiment of the present invention, these active, shiftable masses are characterized as active, smart masses. This means that they are caused to shift by a control system that senses cable vibrations or oscillations above a threshold level and then activates the shiftable masses in a manner which will effectively counteract the cable or cable stay vibrations or oscillations.
The existence of underwater current induced vibrations in cable stays is a phenomenon that possibly can be counteracted by properly designing the stay cables of a marine structure before it is erected. Unfortunately, there has not, in the prior art, been a practical retrofit solution for off-shore platforms and for the platform""s associated cable stays which are already in place.
The fluid flow cable stay dampers bands, and their method of usage, in accordance with the present invention, provide an effective, cost efficient solution to the problem of air and underwater cable stay vibration. The cable stay damper bands utilize flexible or rigid cable encircling bands which carry embedded or attached tension straps. The cable encircling bands can be placed about existing cables in the field without taking the structure, such as a bridge or an off-shore oil platform out of service and without the need for large amounts of specialized equipment.
The damper bands can be either passively or actively controlled. For actively controlled bands, they can be fixed external to the cable stay, producing a xe2x80x9cribbedxe2x80x9d surface, or can be embedded within the cable stay, leaving a smooth surface.
The system of fluid-dynamic and mechanical damping of cables with active smart control, in accordance with the present invention, provides superior damping of cable stay vibration with less cable fatigue. It also will reduce the number of required air or underwater damper bands or rings required for each cable. The active smart control system of the present invention is directed primarily for use with a distributed air or underwater ring or band system. It is also usable for a computer-controlled single point mechanical damper system which could be used either by itself, or in combination with an underwater ring or damper band system.
For a distributed mitigation device, such as the underwater rings in accordance with this invention, it is possible to completely solve the vibration problem by installing the rings only on a partial length of the cable-not along the full cable length. A distributed damper band system will be effective in eliminating significant vibrations in all vibration modes.
Damper bands of the present invention have been found to be very effective when applied to an existing cable stay in air using a pattern of bands placed along the cable at a spacing of preferably twice to four times the diameter of the cable. A similar spacing is effective underwater. Due to the density of water compared with air, it is possible for a cable in, for example an ocean current of 5 mph to vibrate similarly to a cable in air subjected to, for example, 50 mph winds. While a band spacing of two to four times the cable diameter will result in the use of a large number of bands, the number of these bands is nowhere near the number suggested in the prior art as being required for applications in air. The flexible or rigid bands can be installed effectively using uncomplicated techniques so that minimal disruption to the structure during damper band installation will occur.
Unlike prior proposed solutions, the cable stay damper bands of the present invention are not particularly conspicuous, do not require adjacent cables to be connected together, are durable and require essentially no maintenance, other than the possible need for removal of marine growth in the situation of underwater placement of the cable and bands. The damper bands are not apt to add a great amount of weight to the cable stays to which they are attached. Installation of the damper bands or rings in an underwater application is likely to benefit from the use of a robotic installation systemxe2x80x94especially for any extreme depth installation. The cable stay damper bands, with or without active control, and their methods of use, in accordance with the present invention, in diverse situations overcomes the limitations of the prior art solutions. They represent a substantial advance in the art.